Synchronizing system



Fell 20, 1945- F. R. DENNls ETAL 2,369,663

sYNcHRoNIzING SYSTEM I Filed Feb. 28., 1942 2 sheets-sheet 2 F RE QUE NCF OSC/l. l. TOR

f. R. 05m/s /NVENORSE R Fam, JR.

mmdr

A. F. Pomeroy, 2,288,025, a means for maintaining a fixed frequency diierenceis disclosedcomprising la sep- 4.arate oscillator which .is maintained at .a rire,- vquency'v which vis a fliedwdifference from' a given ingf'from' thequencysourcea voltage of'y frequency equal to Patented Feb. 20, 1945 Fred R. Dennis,

Lynahurst,-. anufndwi Panelen,

Jr., Chatham, NQJ.;4 assigno'rs:tofBellxTelephone r Laboratories,Incorporated; New

corporation of New .York f vApplic-ationfFelini'al1-y 2s, miaserial No. 432.862

`6 claims. (orso--ssi Thisfinvention relates torsynchronizing'u sys-.A

temsandmore particularly4 to that type of synchronizing systemv wherein` an alternating cur-.-v

rent generator is maintained at a frequency which isa predetermined difference from a given refer'ence` frequency.

Heretofore in the prior artfii'ced frequency differences Wereusually obtained by modulatingA the given reference frequency with the desired frequency difference and selectingthroughsuitable filter. networks the, sumi or difference frequency.

Howeverin certain cases,.particularlyin applica# f tions involving kprecision measurements, `this method producesan outputA containing a number 'of troublesomepnwanted frequency components V.which render the measurements unreliable.

lIn a patentappucation filed August 3, 1940,1by -Serial' No. 350,298,y .now Patent reference frequency.` Thisrtype of circuit effecztively isolates the input circuit of. reference frequency froml the output -circuit andr thereby vgreatly improves the purityfof the. output de- Vlived-.from the system.A f j i;

It is the ob'ject ofthis invention tofurtherim'- `prove-'the purity of the Youtput frequencyby producing anoutput frequency reference frequencybya givenamount and which output' is f substantially entirely devoid of "any differing from 'a Fig. j3.',s l1ovv's inmore detail the circuit of the phasejdiscriminator rectifier and oscillator .control system of Fig. l;

Eig. Llis a vector diagram illustratingV theprinfjciplesof operation of theiphasel discriminator;

i. Fig. 5 is a block diagram showingamoclification .of the invention; and "Eigljgis another block diagraml showing a still furtherlmodication. i Referring j now more particularly .to l'fig.f l'; in

which referencenumeral I is an oscillator. Whose` frequency Yisjtcibe controlledand maintained at aiixed frequency .difference AF from theinputl voltage of reference frequency F.

.The otput'yoltage 'fr is combined lwith the reference requencyfF `in a .modulator v2 `of conventional .design...` AThis modulator preferablycontains asuitable'lter for selecting! the difference frequency .AF ywhich dif Vference frequency-ismultiplied -an integral `numli berof timesby av frequency VVmultiplier ,3. i Moducomponents equal either tolrthereference fre- `quency or the maintained difference frequency.

The foregoing object is attained by this inven- -tion'rwhich provides-a voltage vsource ofr'eferencefrequency,van oscillator producing an output fre- `quency nominally "differing f from thefireference frequency by a -given amount,` means for lderivoscillator and the referencefrean integral multi-ple ofthe differenceibetween theirY frequencies, a control voltage source `the frequency Whereofis substantially equal to the i ymultiplied differenceffrequency, andfrneansA re-` sponsive to the `control voltage f and thevoltage of multiplied Vdifference,frequency vfor controlling the-'oscillator frequency. A i

` Theinvention maybe better understoodl by ref ferring to thedrawings, in which: f

lFi'g. 1 -isf` a block 'diagraml showingl the-essential elements rofzone` embodiment of this invention;`

Fien 2 iszyanother block diagram showing how amplifiers may be addediadvantageously'to:the.

lzors. and frequency lmultiplierssuit'able for the practice of. this! invention` are wellkno`wn1tothe yart andrequire Ynodetailed description.` `A. voltage lcfv constant frequency, Which-ispreferably vacrystal controlled. 'osc`llator, produces. an outputjfvoltagefcf y'frequency FD.. vThis .constant freiquency. voltage `1s 1 combined 1in .a discriminator `5 Various typesofdiscriminators and rectiers-may ,beutilized forthe practiceof this invention, one sugliV beingdescribed in more detailin connec- .tionwith Fig. 3. I*This discriminator-and rectifier isladapted toifproducefaY direct current .voltage .varying in magnitude andpolarity-in proportion tothe phase .difference between lthe voltage of multiplied v,frequency and the voltage from .-.the .constant frequencysource. As more particularly hereinafter described in connection with Fig.y 4

1-,bedirect current output voltagewill be zero so- .long asfth'e voltage'ofmultiplied frequencyvmaintainsstrict quadrature with the.v constant fre- .quen-cy voltage'. -zShoulda phaseishift occurone Wayaor the other dueA to any cause whatsoever, a, directy current :voltage will Abe, produced which `will 1ibe` ,proportionalf'tothe magnitude-and direction'. ofv ideparturei `from quadrature -This direct `current voltage.- is utilized :to :control 1 the 1 frequency Yof .oscillatoryy I andrthe circuiti constants areyso yselecte dthati 'shouldthe frequencygof-- either :oscillatory-.I-` orA the-:reference frequency rF tend jtoi change slightly it=wi1l cause afmomentaryfshift in' "phase :between the i multiplied 4frequency Vvoltage and the.-fconstant;` frequency. voltage which .-is

sufcientztog produce a .direct lcurrent voltage `of.

of oscillator. I of'frequency proper magnitude and polarity to correct the frequency of oscillator I and keep it locked at a constant frequency difference from the input source of reference frequency F.

In Fig. 2 the circuit arrangement is substantially the same as that shown and previously described in connection with Fig. 1. In this figure, however, amplifiers 6, 1, 8, 9 and I0 have been included to show locations where they may be most advantageously used. In this figure as well as in Fig. 1 it should be noted that the modulation products from modulator 2 are by reason of the process of frequency multiplication and the use of the discriminator rectifier very thoroughly isolated from the output of oscillator I so that the output from amplifier I is a substantially pure sine wave of frequency f. l l

Fig. 3 shows in greater detail the preferred circuits for oscillator I and the phase discriminator rectifier 5. In this figure the voltage of con` stant frequency Fn coming from amplifier 9 is fed into the phase discriminator network 5 by way of the primary of transformer II. The secondary of this transformer comprises two substantially identical windings, serially connected at point I2. The free ends of these two windings are connected tothe plates of two diode rectiilers I4 and I5. Equal resistors I8 and I9 are serially connected between the cathodes of these two diodes and their intermediate junction is connected to junction' I2 through an inductor I6. A differential condenser I3 is connected with its stationary plates to the anodes of the two rectifier tubes and with its movableV plate to ground. A capacitor 20 is connected across the two serially connected resistors I8 and I9.

It is clear that, assuming perfect symmetry and zero output from amplifier 8, the rectified voltage drop across resistor I8 will be equal and opposite to the rectified voltage drop across resistor I9 so that the net voltage applied across the terminals of capacitor 20 is zero. Differential capacitor I3V is used for achieving this balance and to overcome slight unsymmetrcal conditions.

The multiplied frequency difference NMF) coming from amplifier 8 is applied to the phase discriminator network at junction I2. An alterhating current path may be traced through the vnetwork from junction I2 through inductor I 6, capacitor I'I and to ground 2| through which the circuit returns to amplifier 8. Under normal cirto become less than 90 degrees as shown in Fig. 4 whereupon voltage e2 becomes greater than voltage e1. This results in the rectified component voltage e2 becoming greater than the rectified component voltage e1 so that the lower plate of capacitor 20 becomes positive with respect to its A upper plate. Should the frequency tend to shift cumstances the frequency of the multiplied difference frequency from amplifier 8 is equal to the constant frequency Fo coming from amplifier 9 and these voltages have such a phase ref lationship that the voltage drop V produced across inductor I6 is in quadrature with the secondary voltages E2 induced in the secondaries of transformer I I.

Referring now to Fig. 4 it will be seen that the vector difference between the voltage V and voltage E2 is equal to the voltage e1 and the vector sum of voltages V and Ez is equal to the voltage e2. So long as the phase angle o existing between voltages V and Ez remains 90 degrees, the

resultant voltages e1 and e2 will be equal in magm- 'tude. Since their respective rectified components e1' and e2 are connected in opposition through resistors I8 and I 9, as shown in Fig. 3, their rectifled resultant voltage appearing across capacitor -20 remains zero. If, however, as hereinbefore stated there should be a slight shift in frequency of loscillator I or of the input reference source 'it would amount to a phase shift between voltage V and E2; This may, for example, cause angle' gb in the opposite direction so that angle qb in Fig. 4 becomes greater than 90 degreesr the upper plate of capacitor 20 would become positive with re spect to its low'er plate.

The voltage appearing across capacitor 20 is applied to the control grid of a control tube 23 through a resistor 22 as shown in Fig. 3. The action of this control tube is well-known in the art, as the principles of its operation were described in the Proceedings of the Institute of Radio Engineers, October 1935, page 1135. Its operation, however, in connection with the tank circuit 2l of the oscillator will be described briefly for the purposes of this specification. l

Tube 28 is a part of oscillator I, the circuit whereof may be of any well-known form. The tank circuit 2'I comprises an inductor L and a tuning capacitor C formed by two series-connected condensers 29 and 29'. Condenser 35 is a by-pass condenser and resistor 30 supplies the direct current voltage from the direct current power supply to the plates of the control tube 23 vand oscillator .tube 28.

25 is made relatively large compared with the reactance of capacitor 26'so that the voltage drop across the capacitor 26 lags the voltage across the tank circuit by nearly 90 degrees. The voltage drop across capacitor 26 is applied to the control grid of control tube 23 causing this tube to take current substantially in synchronism with the current through coil L of the tank circuit 21. This causes the control tube 23 to appear as an inducftive reactance to the tank circuit of oscilla- .tor I. The magnitude of the current taken by the control tube is controlled by the direct current lbias on itsA control grid which is obtained from capacitor 20 through resistor 22 as hereinbefore described. Consequently the voltage appearing across capacitor 20 in the phase discriminator rectifier network 5 will control the frequency of `more positive with respect toits lower plate the oscillator.

The connections throughout the system are so made that should, for example, the frequency of the oscillator under control tend to decrease sliglhtly it will cause voltage vector V to move in the positive direction thereby increasing the phase angle qi existing between this voltage V and voltage E2. As previously described this will cause the upper plate of capacitor 20 to become which in turn causes the control tube 23 to take a larger current and appear as a smaller inductance to the tank circuit 2T. This effective decrease in inductance will cause the frequency of the oscillator under control to increase'to its original value. Similar considerations will show quency F tends` to shift.

In Fig. 5 the invention is shown'in 'a slightly the 'control effective when the reference freusedy in vmeasuring of this frequency multiplying feature these sys- 60 tems'have been found vvery unreliable for pre-a` escasas fuifrerentironn. 1 un case' instead' of f .first fcliferencev between the reference frequen'cyl v'comingfrom #amplifier "G-andimultiplying this diierenceffrequency,*these twofrequencies are "first multiplied separately by vthe same amount vlflyfse'rparate multipliers L33 and 341-` v'The multipliedfdiiference frequencylwhich fis {equal-to N(AF) is 4applied directly 'to :the ydiscriminator otherwi rectifier `'network 5. The operation issubstantially identical to that :describe fo -F igs'.1land`i2 above. The advantage ini-using,'the-twof-frqency multipliers in `the mannerl described is to further isolate thevoltage of reference frequency `F and the voltage -of vdifference frequency AF from the output of oscillator I.

It should be noted that reference frequency F. close frequency control within a fracion of a cycle, the frequency of ,oscillator l being locked l to a fixed frequency difference withrespect vto source F, any attempted departure amounting' only to a phase shift.

Itis sometimes Vdesirable tobe able tov quickly change the difference frequency AF. In Fig. 6

oscillator 4 is shown variable to 'provide an easy V4 so that the desired difference frequency AF is a closely'controlled function of the frequency c f oscillatorv 4.- AThe accuracy of controlwill be such that the desired difference [frequency AF will FD divided by the be equal tc the frequency multiplying' factorN. l

While the invention nection with' a preferred form of phase type discriminator, other types of discriminator if properly designed have Abeen' found operative and may be substituted. For example, .the frequency States Patent issued vSeptemllery .2, 1941, toxE.E

Felon, or the frequency discriminator disclosed discriminator disclosed in .United 2,254,601,

in'the Proceedings of thev Institute oifvRadio iEngineers,l October 1935page- 1127, may be used.

However, whereexactfrequency control is re-j vdiscriminator such as the one herein disclosedfis preferred lover the tuned quired the vphase type circuit type. Y.

VIn actual applicationsthese systems have been found very effective for Iisolating the unwanted the output voltage inL modulation products from highly sensitive self-tuningdetector networks systems. Without the use means including atleast lone frequency multiplier having a fixed multiplyingfactor for deriving from the oscillator and the reference frequency source a voltage of frequency equal toa multiple of the difference between their frequencies, a

illalt'or"-l" `'under control and thel'lyoltage sourcev by increasing the vmagnitudes of the multiplying factor N .the sensitivity of control is proportionately increased kbecause a phase shift of @between voltages 'V and E2 will result-fromy a phase shiftf ib/,NY between thefvoltages `o-f .oscillator I andthe This providesextremely has been ldescribed in conyvoltage; source of fzconstant frequency 'exactly #equal- @to the'product of the# said desired 'i pre-,

l lideterminedz-flxedf frequency "difference multiplied by saidlffixed',xnultiplying'factor,` an *electronic 5 yphase discriminatorfhaving. v two'alternating1 cur- @rent v.inputs `and* f a f direct. current youtput circuit and'of thetype wherein, the direct current out"- put. voltage;

:connecting the said voltagerof constant .fre#

- quency-toll the other alternating; current input '15 circuit and circuits connectingthe 'direct cur- 'rent output circuit to said oscillator whereby said oscillator is maintained locked with said reference frequencysource but at-a frequency dify fering therefrom exactly equal to said'predetermined fixed frequency difference.

2. The combination according to claim 1 includes meansvfor adjusting said constant frequency to anydesired predeterminedvalue.

3. Inr asystem for automatically `maintaining an oscillator electrically locked at any desired predetermined fixed frequency difference from a reference frequency source the combination cornprising a voltage source of reference frequency,

an oscillat to an applied direct current voltage, means for derivingvfromfhe.oscillator and the reference frequency source a voltage of difference frequenlj exactly equal to the product 'of the said desired predetermined fixed frequency difference multi-` plied'by saidnxed multiplying factor, an elec- 40 tronic lphase discriminator-having twoalternatl ing current input circuits and a directcurrent outputcircuit and ofthe type wherein the direct `current outputvoltage is proportional to the `phase angle between'I the two alternating current input voltages, circuits connecting the said voltage `of multiplied different frequency to one of said alternating current input cuits connecting the said voltage of constant frequency tothe other alternating currentk input `circuit, and circuits connecting .the direct currentvoutput circuit to said oscillator whereby said oscillator is maintained locked with Said reference frequency source but at a frequency differing therefrom exactly equal to said predeter,- mined fixed frequency difference.

wherein said voltage source of constant frequency includes vmeans quency to any desired predetermined value. c fr.` In a system for automatically maintaining an `oscillator electricallylocked at any desired prising a voltage 05 van oscillator controllable in frequency in respense to an applied ldirect current voltage,'a frequency multiplier for multiplying the frequencyof the-reference frequency voltage by a fixed multiplying factor, another frequency multiplier for multiplying the oscillator frequency by a fixed multiplying factor exactly equal to said first-named fixed multiplyin-g factor, means deriving from the two multipliers a voltage of difference frequency which frequency is equal to the difference @between said reference frequency and is v'proportional tothe ,phase angle` f between `the two-alternating current inputvolt- 'theflsaid `voltage .of y

wherein said voltage source of constant frequency r controllable in frequencyV in response circuits, other cirf 4. The combination according to claim 3` for adjusting said constant fre- Apredetermined xed frequency difference from a reference frequency source thecombination comsource of reference frequency,A

'said oscillator frequency multiplied by said fixed multiplying` factor, a voltagesource. of constant lfrequency exactly equa1 to 4the product of the -said desired predetermined, fixed frequency difference multiplied by said fixed multiplying factor, an-electronic phase disc'riminator havingtwo alternating current input circuits and a direct current outputA circuit and of the type `wherein the direct current output voltage is proportional to the phase angle between the two alternatingA current input voltages,` circuits connecting the said voltage of multiplied difference frequency to one of said alternating current input circuits, other circuits connecting .the said voltage of constant frequency to the other alternating current .input circuit, and circuits connecting the direct current output circuit to said oscillator whereby said oscillatoris maintained locked with said referencey frequency source but at a frequency differing therefrom exactly equal to said predetermined fixed frequency difference.

6. The combination according to claim' 5 Vwherein said voltage source of constant frequency includes means `for adjusting said constant frequency to any desired predetermined value.

i FRED R. DENNIS.

EDWIN P. FELCH, JR. 

